Despite sensitive radioimmunoassay tests for HBsAg applied to all donor blood, some type B hepatitis continues to occur following transfusion. It has previously been suggested that HBsAg-negative individuals that have anti-core antibody may transmit the hepatitis B virus. The availability of a new sensitive radioimmunoassay for anti-core antibody allows this postulate to be tested in our prospectively followed open heart surgery patients. Current tests also allow for determinations of IgM versus IgG anti-core antibody and the clinical significance of these antibody subclasses will be also investigated.